1. Technical Field
The present invention relates to a pulse detector and a pulse detection method.
2. Related Art
A pulse detector is a device for detecting the pulse originating from a human heartbeat. This device removes a noise signal component (motion affected signal) generated due to the effects of a motion of the human body from a signal (pulse wave signal) from a pulse wave sensor worn on an arm, a finger, or the like and detects only the signal (pulse signal) originating from a heartbeat.
In a pulse detector of a type in which an optical pulse wave sensor is worn on a finger, a wrist, or the like, since changes in the bloodstream can occur due to a motion of the finger or wrist itself or an impact near the finger or wrist, a noise signal can be input to the pulse wave sensor. This noise signal has higher signal level than a heartbeat component signal and is a great hindrance to the measurement (frequency analysis) of pulse. Such a noise should be removed as completely as possible. Particularly, in a pulse detector that measures the pulse continuously (every several seconds) in the course of daily activities and exercise, limitation to use under conditions where “the fingers may not be moved or touched” may greatly degrade its usability.
FIGS. 14A and 14B are graphs showing a noise signal in pulse wave signal data. Curves 120p and 120q on the top of each graph show the waveforms of pulse wave signal data, and bar graphs 122p and 122q on the bottom show the results of frequency analysis by fast-Fourier transform (FFT). FIG. 14A shows a state where a noise signal is not superimposed on pulse wave signal data, and FIG. 14B shows a state where noise signals 144 and 146 are superimposed on pulse wave signal data. A sensor output signal that cancels the heartbeat component is generated by an impact to the vicinity of the pulse wave sensor and a motion of a finger or the wrist. The low-frequency component of this large power signal may lead to an error in the pulse detection.
A sensor such as a pulse oximeter that optically acquires changes in blood volume is normally required to be worn at a location, such as the fleshy side of a finger, the palm, or a nail, where a large volume of arterial blood flow appears near the skin. Therefore, in many pulse detectors of the related art, a technique in which a pulse wave sensor is mounted at such a position as described above is proposed (for example, see JP-A-2005-198829). Moreover, when an external sensor and sensor cable are eliminated, and the sensor is embedded in a device body, the usability thereof increases.
Moreover, JP-A-2007-054471, for example, discloses a technique in which a pulse detector includes a plurality of band-pass filters and removes a noise component by subjecting signals obtained from a pulse wave sensor to filtering by a band-pass filter that passes a frequency signal near the frequency of the present pulse.
According to the technique disclosed in JP-A-2005-198829, the pulse wave sensor is worn on a finger, the palm, the wrist, or the like, particularly, where a motion occurs more frequently than other parts of the human body. Therefore, when a user wearing the sensor moves a portion near the hand, changes in the bloodstream occur different those in the bloodstream caused by a heartbeat as noise caused by a motion of the hand. The changes are input to a signal caught by the pulse wave sensor as noise. Thus, there is a case where the presence of this noise becomes a hindrance to analysis of pulse frequencies.
Moreover, when the user wearing the sensor touches the position in which the pulse wave sensor is worn or the peripheries thereof with an object or another part of the user's body, changes in the bloodstream occur differently from those in the bloodstream caused by a heartbeat as noise caused by a touch on the hand. Since the pulse wave sensor catches the changes in the bloodstream, the changes are input to a pulse wave sensor output signal as noise. Thus, there is a case where the presence of this noise becomes a hindrance to analysis of pulse frequencies.
The above-mentioned problems have a high degree of influence on a device in which the pulse wave sensor is embedded in a device body. The following can be thought of as the reasons thereof. Bones such as the radius and ulna, tendons, and muscles come together in the wrist, and changes in the bloodstream occur when the shapes of the tendons and muscles are greatly changed with the movement of a finger, the hand, and the wrist. Looking into the flow of arterial and venous blood, the arterial blood exhibits clearer changes in the bloodstream with a heartbeat than the venous blood, and accordingly, the rhythm of a heartbeat appears more clearly as a pulse wave sensor signal. The heartbeats are rarely detected in the venous bloodstream. However, since the subcutaneous tissue on the outer side of the wrist includes few arterial blood vessels (or they are located at a deeper side), changes in the bloodstream caused by external factors are more dominant than the changes in the bloodstream caused by the heartbeat when the bloodstream is caught by the pulse wave sensor. Therefore, it may be difficult to detect the changes in the bloodstream caused by the heartbeat when a motion of the hand or an impact to the peripheries of the hand is input to the sensor.
Moreover, according to the technique disclosed in JP-A-2007-054471, execution of such processing on a hardware circuit must involve many determination processes based on IF statements, which may increase the processing time and load. Therefore, the use of this technique in a pulse detector having a size as small as a wristwatch is not desirable from the perspectives of processing ability and consumption power.